aotcompile: destroy LLVM context after serializing combined module

[xal-0]

We already save some memory here by deleting the jl_native_code_desc_t after
we're done serializing the combined module, but some data in the module's
LLVM::Context lives on until the end of the scope in jl_dump_native_impl.
Once we're done with the module, move the lock and ThreadSafeContext so the
reference count drops to zero.

A crude measurement shows that when compiling the Base sysimage, about 3 GiB is
in use. Deleting the jl_native_code_desc_t (as before) saves about 600 MiB,
and cleaning up the context saves an additional ~500 MiB.

[vtjnash]

We might be able to use modern C++ features for this (though it might require replacing std::function with llvm::unique_function, which hasn't yet been standardized, so that it compiles to do the right thing)

Suggested change

	data_outputs = compile(*dataM, "text", threads, [data, &lock, &TSCtx](Module &) {
	// Delete data when add_output thinks it's done with it
	// Saves memory for use when multithreading
	auto lock2 = std::move(lock);
	delete data;
	// Drop last reference to shared LLVM::Context
	auto TSCtx2 = std::move(TSCtx);
	});
	data_outputs = compile(*dataM, "text", threads, [data, lock = std::move(lock), TSCtx = std::move(TSCtx)](Module &) {
	// Delete data when add_output thinks it's done with it
	// Saves memory for use when multithreading
	delete data;
	});

[xal-0]

I think we'd need to explicitly invoke the destructor for the lambda: godbolt link

[vtjnash]

I think that is the design mistake llvm::unique_function may fix (over std::function)